In `/src/applications/project/query/PhabricatorProjectSearchEngine.php`, change the two only uses of `PhabricatorSearchIntField` to `->setIsHidden(false)`. Then go to `/project/query/advanced/` and enter example values into the `Maximum Depth` and `Maximum Depth` fields and click the Search button.
Example values: `13`, `0`, `-13`, `foobar`, `foo13bar`, `-foo1313`, `13.5`, `-13,5`, `13,5`, `-13,5`, `-`, `-0` (the latter is still accepted as `0`).
It seems that there are no API calls to test - `/conduit/method/project.query/` does not offer passing depth values.